SREL Reprint #0368




Biochemical Genetics of Dunfish. I. Geographic Bariation and Subspecific Intergradation in the Bluegill, Lepomis macrochirus

J. C. Avise and M. H. Smith



Electrophoretic variation in proteins encoded by 15 genetic loci was analyzed in 2415 bluegill (Lepomis macrochirus) representing 47 populations from 7 Southern states.  Populations from the Florida peninsula and southeastern Georgia (L. m. Purpurescens) differ in allelic composition at several loci from populations in central and western Georgia west to Texas (L. m. macrochirus), yielding coefficients of genetic similarity below the range generally found for continuously distributed conspecific populations in other vertebrates, but quite comparable to previous reports for various semispecies pairs.

Populations of L. m. purpurescens are essentially monomorphic for Es-3100 and Got-258, while populations of L. m. macrochirus are segregating for Es-396 and Es-398, and are fixed for Got-2100.  Within several river drainages in South Carolina and eastern Georgia, bluegill populations are segregating for all of these alleles.  In particular, a highly significant correlation between frequencies of Got-258 and Es-3100 indicates that the two subspecies are intergrading in a wide zone of overlap.  A closer examination of genotypic class proportions of a large population of bluegill from the intergrade zone confirms that the two subspecies are backcrossing and area apparently fully interfertile.

Degrees of introgression appear equal for alleles at these loci.  The high correlation ion population allele frequencies across loci is compatible with the hypothesis that the alleles are behaving as neutral markers of intergradation.  However, mildly significant deviations from expected genotypic proportions may indicate influences of selection.

The pattern of intergradation evidences a secondary meeting of allopatrically evolved races.  Since populations of pure L. m. purpurescens are largely confined to the Florida peninsula, it is likely that Pleistocene rises in sea level were important in their original isolation from L. m. macrochirus.

Populations of bluegill within reservoirs are generally homogeneous from frequencies of common alleles at polymorphic loci.  However, there is significant heterogeneity in allele frequencies between reservoirs within any drainage system.  The magnitude of this variance is greatest in the intergrade populations within the Savannah River basin, and is far less in “pure” samples of L. m. macrochirus.  The bluegills examined may be characterized by three areas of relative regional uniformity, in which genetic differences within a drainage system are probably as great as those between drainage systems:  (1) the Florida populations of L. m. purpurescens 2) the intergrade populations and 3) populations of L. m. macrochirus.

 Keywords: Par Pond Reservoir System

SREL Reprint #0368

Avise, J.C. and M.H. Smith. 1974. Biochemical genetics of sunfish. I. Geographic variation and subspecific intergradation in the bluegill, Lepomis macrochirus. Evolution 28(1):42-56.

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